There exists a continuing need for improved mufflers for vehicles for noise reduction with a minimum of back pressure on the engine. Engine performance and fuel efficiency are directly related to the back pressure at the engine exhaust system. The lower the value of the back pressure, the greater the engine efficiency. Unfortunately, back pressure is necessarily added by the presence of a muffler. No place is an unmufflered internal combustion engine acceptable so engineers have learned to live with the efficiency reduction caused by the muffler.
Various forms of chambering of the muffler are known, one including two chambers, an initial one in which the exhaust gases are introduced via a perforated pipe for transfer via the open chamber to a second perforated pipe to a second chamber which is filled with acoustic energy absorbing material and then out through the last pipe. Significant sound reduction is obtained in such mufflers and the industry accepts the efficiency limitations imposed by such mufflers.